Electroless or autocatalytic coating of dielectric surfaces is a well known process finding wide-spread utility in the preparation of such diverse articles as printed circuits, automotive trim, mirrors, etc.
Normal commercial electroless coating processes generally involve an initial cleaning and etching of the dielectric substrate by physical or chemical means to improve adherence of the metallic coating. The etched surface is then catalyzed by suitable catalytic compositions and processes to provide a surface capable of electroless plating initiation. In the prior art, the catalytic treatment generally encompassed the use of precious metals. More recently, compositions and processes utilizing non-precious metals have been disclosed suitable for electroless plating of dielectrics. U.S. Pat. Nos. 3,993,491, 3,993,799, 3,958,048, 3,993,801, 3,993,848, 4,048,354, 4,082,899, 4,087,586, 4,131,699, 4,136,216, 4,132,832, 4,150,171, 4,151,311 and applications Ser. No. 941,044 now U.S. Pat. No. 4,180,600, 803,777 now U.S. Pat. No. 4,181,760, 661,663, 893,248 now U.S. Pat. No. 4,167,596, 934,344 now allowed, 817,242 now U.S. Pat. No. 4,181,759, 938,438 now abandoned and 938,890 which are included herein by reference disclose the prior art as well as the recent advancements in which non-precious metals have been reported. Of these it should be noted that in U.S. Pat. No. 3,993,491 (Example 31) a prewet solution was used prior to the catalytic composition which was found particularly useful in through-hole plating of copper laminate, especially after the persulfate etch step. Specifically, using the prewet step has insured a greater adsorption of the catalytic composition onto the laminate surface. This modification nullifies the deterioration in adsorption (and/or absorption) characteristics resulting from the etch step. Also, in U.S. Pat. No. 3,993,848 the use of a linking agent is shown which can be made operative in either of two basic modes, but in each case it permits a greater adsorption of the active catalytic component(s) present within the catalytic composition. U.S. Pat. No. 4,087,586 demonstrates the use of colloids based upon insoluble compounds of non-precious metals and particularly those of copper, nickel, cobalt and iron.
It is also recognized that at least certain of the primer compositions in U.S. Pat. Nos. 3,993,491 and 3,993,848 are in reality of a colloidal nature. Specifically, those compositions which comprise of tin(IV) ions are expected to yield a colloidal nature due either to the insolubility of this ion or to its hydrolysis reaction to .beta.-stannic acid. The copper(I) ions being part of a tin(II) complex are also affected by the adsorption characteristics of the tin(IV) and any colloid resulting thereof.
In reviewing the teachings disclosed in U.S. Pat. Nos. 3,993,799 and 3,958,048 it is evident that colloids of either hydrous oxides, metals (elemental state) and alloys (phosphides, borides, etc.) are useful in the catalytic treatment either as a two step or a single step activation treatment. Generally speaking, preferred non-noble metals in the above colloids are selected from the metals of copper, cobalt, nickel and iron and mixtures thereof, although as suggested in U.S. 3,993,799 other nonprecious metals may be used. It is recognized that it is generally desirable to have suspensions (dispersions) of very fine particulate matter for both stability (i.e., against precipitation), reactivity, and adhesion to the substrate. Accordingly, it is highly desirable to prepare such suspensions under conditions which would yield finely divided and highly stable colloids.
It was also recognized in U.S. Pat. No. 3,993,799 that those fine sized colloids of the metals and alloys due to their contact with water and/or air would react to provide a surface oxide and hence they were classified as hydrous oxide.
This type of problem has not been encountered in the prior art which has used precious metal colloids due to their inertness towards oxidation. It is recognized that if the surface of the colloids of metals or alloys is oxidized, the induction time would be prolonged when contacted with the electroless plating bath; this is true in particular whenever a single stage activation step is carried forth. Accordingly, it is highly desirable to provide means by which the formation of surface oxide for the metals or alloys is eliminated or minimized. The present invention provides compositions and processes which improve upon these disadvantages.
It is also well recognized in the art of electroless plating that for effective electroless plating onto catalytically treated non-conductors at least one of the following requirements must be met:
Case I: The catalytic surface may react chemically with the reducing agents present within the electroless plating bath. More than one chemical reaction may take place.
Case II: The catalytic surface may react chemically with the metallic ions present within the electroless plating bath in a galvanic type replacement reaction.
Case III: The catalytic surface may react at first with other chemical component(s) present within the electroless plating bath, e.g., complexing agent.
In Case I the chemical reactions may include chemical reduction of the catalytic components present on the dielectric, and/or decomposition of the reducing agent at the interface ultimately yielding hydrogen gas via an active reducing agent intermediate. In Case II to permit a galvanic replacement reaction it is recognized that some of the metal ions present in solutions must be more noble with respect to the metal and metal ions present on the treated non-conductor surface. Such relationship is well recognized from the EMF series. Thus, while metals like copper, cobalt, nickel and iron may be preferred as recognized in U.S. Pat. No. 3,993,799, other non-precious metals may also be of potential use (e.g., zinc, manganese, aluminum, etc.).